The invention pertains to devices for sensing difference in height between two substantially horizontal surfaces.
In order to construct machine tools or set machine tools up for operation, it is frequently necessary to ensure that two horizontal surfaces are at exactly the same level. Examples of such surfaces include the load bearing surfaces of a set of horizontal machine tool ways, and surfaces on machine tool beds. In some instances it is necessary to insure that two separated surfaces are level to within 0.0001 inches.
In the past, such leveling tasks have been performed by devices which include two chambers, a flexible conduit connected between the chambers, and an amount of mercury which partially fills the chambers. The chambers are respectively placed on the surfaces, whereupon mercury flows between the chambers through the conduit in response to gravitational force. The mercury levels in the two chambers thus gives an indication of the height of each of the surfaces. Additional apparatus provides an indication when the heights are the same or, in some cases, shows the value of a height differential.
In one class of such prior art devices exemplified by U.S. Pat. No. 4,379,367, issued Apr. 12, 1983, for an "Electrically Controlled Level," the mercury is mechanically coupled to level indicating structure by means of floats, diaphragms or pistons which directly contact the mercury to alter the resistance or capacitance of an electric circuit. However, the use of mechanically movable parts in a leveling device tends to limit measurement sensitivity and may increase cost and complexity. Also, such device is vulnerable to wear of its moving parts.
In another class of prior art devices exemplified by U.S. Pat. No. 3,645,135, issued Feb. 29, 1972 for a "Mercury System", a capacitor plate is fixed above the mercury in each chamber. The distance between the capacitor plate and the mercury level in each chamber is then determined by measuring the capacitance therebetween. However, such devices tend to require extremely vulnerable electronics, making them very vulnerable, for example, to noise problems.